Industrial furnaces using powdered coal or cokes are used, e.g. in thermal power stations or thermal power plants for electricity and/or heat generation. Furnaces using powdered coal or cokes are also used for cement kilns. The use of coal is more common than that of cokes in furnaces for power plants; cokes are more often used in blast furnaces and either one is commonly used in cement kilns.
A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated. The greatest variation in the design of thermal power stations is due to the different fossil fuel resources generally used to heat the water. Certain thermal power plants also are designed to produce heat energy for industrial purposes of district heating, desalination of water, cement kilns or the like, sometimes in addition to generating electrical power.
Many thermal stations use coal as the main fuel. Raw coal is transported from coal mines to a power station site by trucks, barges, bulk cargo ships or railway cars. The coal received at site may be of different sizes. The coal can be used as such, or can be conveyed to crushers which crush the coal to about ¾ inch (19 mm) size up to about 1.2 inch (30 mm) or the like. The raw feed coal from the coal storage area is optionally crushed, and conveyed to the coal feed hoppers to the mills (pulverisers) at the boilers.
The coal is next pulverized into a very fine powder. The pulverizers may be roller mills (rotating drums grinders, or bowl mills), ball mills, or other types of grinders. Grinding and milling are generally used as synonyms, and these words are used in the present specification as synonyms as well.
Generally, bituminous coal is used, but the furnace can also be fired with powdered cokes or other qualities of coal. Bituminous coal is softer than cokes.
Globally, fossil fuelled thermal power plants produce a large part of man-made CO2 emissions to the atmosphere, and efforts to reduce these are varied and widespread.
One of the efforts comprise using organic material such as wood saw dust, or wood pellets, sewage sludge pellets and the like as fuel in power plants and kilns. However, such co-firing is substantially limited by a number of factors. A relatively recent publication (Co-firing wood pellets with coal; in World-Generation June/July 2011, Vol 23 number 3) explains that co-firing can be done in a number of ways: (i) indirect co-firing; this means that the additional fuel is prepared and burned separately, and the heat is combined with the heat of the primary source. Obviously, this is a costly method, as a complete additional furnace has to be build. (ii) direct co-firing with separate injection; this means, that the fuel is prepared for burning, e.g. by separate milling, and that the fuel is added to the flame with a separate lance. This requires additional lay-out and investments, yet, this is (often) economically feasible; (iii) direct co-firing without separate injection; In this case, the additional fuel is mixed with the optionally crushed (but not yet powdered) coal, and the fuel stream is directed to the milling station to prepare the powder for the furnace. This last mentioned method is considered a preferred method in view of costs. However, it is generally accepted that this method cannot be used with most secondary fuels, or, for example for saw-dust, for a maximum of a few percent only. Problems generally are considered to reside in the milling operation and/or the burning, yielding not sufficient energy by the secondary fuel.
Comparable information can be found in an article by B. Livingstone “Direct injection advances biomass co-firing in large coal fired plants”; PEI (Power Engineering International), 1 Jul. 2008 (XP055139025). The main focus of this article is the direct injection of ground biomass, in which the biomass is ground in a milling operation separate from the coal milling operation. Further, the amounts suggested to be suitably used seems to be substantially overestimated.
Further, it is known to co-fire plastic containing waste as fluff. This requires substantial investment to handle the fluff, and lowers the efficiency of the burner, as particles are so large, that the burn-out time increases.
In JP 2000/256504, it is suggested to use waste plastic as additional source for grinding with coal. No pellets are described, and therefore the waste stream will consists of irregular forms, much like fluff. The use of fluff severely hampers the transportation of the waste material. Furthermore, the co-grinding of coal with plastic material is performed at room temperature in a hammer mill. Thus, although co-grinding is described, this co-grinding is not under the process conditions of grinding of coal as normally done in power plants. In processes wherein coal is ground, heated gas (usually air) is used, which gas is also used to blow (or transport) ground particles to the furnace. This gas is generally heated till at least about 200° C. or higher (inlet temperature), while the outlet temperature generally is about 70° C. or higher. At such high temperature, it can be expected that the plastic will melt, causing processing problems.
U.S. Pat. No. 6,635,093, WO2008/107042 and WO2010/012510 describe pellets suitable for use in powdered coal fuelled furnaces. To achieve a suitable burn-out time, it is important that the pellets are ground to particles predominantly smaller than 2 mm. Milling is performed in a separate grinder, and an air turbulence mill is indicated. The ground particles are preferably blown into the flame directly. Other references refer to a hammer mill, or a pin mill for grinding pellets. All these mills are high maintenance, and/or (relatively) low in throughput.